Inexperienced users can edit more freely (really? Or will separate metadata add more complications to what they have to learn?)

Parsing the text is simpler (how? doesn't this add more complications, since the parser has to combine another set of data with the page text?)

Recent changes can be identified by the type of change, and users not interested in certain types of meta data change need not bother looking. Also, certain types of changes, e.g. categories, can be followed much easier.

Changes to meta data can be consistently checked before committing (e.g. Categories must exist). Of course, this can be done using the current system too.

The metadata can be stored in a separate table, metadata, with metadata_id primary key. Each time the metadata is revised, a new metadata row is created, and the page.page_metadata is updated with the new metadata_id.

What about older versions of meta data and article data? How should those be combined? Probably when a revision is saved, revision.rev_metadata should be populated with the current page_metadata (i.e. the most recent metadata_id for that page). Then when you view old revisions, it will know what metadata to combine with the archived text.

Store metadata in Wikidata. Users are going to have to use Wikidata anyway for interlanguage links and such, so this is no big deal. There may need to be an InstantData set up (analagous to InstantCommons) as non-WMF wikis' local repositories for data pulled from Wikidata.

Similar to what Extension:Description2 does: strip out sitenotices and such and put the article lead in the page description database field, unless there is metadata overriding this description. There could be short versions and long versions of the article lead. The short version would be one sentence. The long version would be the first paragraph.